High power fiber amplifiers have become increasingly popular due to their high efficiency, simplicity and reliability. In addition, they may be easily ruggedized, due to their simple arrangement.
High power applications generally use a double clad fiber. This fiber comprises a core, usually doped with a lasing material such as rare earth ions or other materials, an inner cladding encircling the doped core, through which the pump power flows and is gradually absorbed in the doped core, and an outer cladding encircling the inner cladding and forming a dielectric wave guide for the pump signal. The optical characteristics of the inner cladding closely match high power diode lasers, commonly used for solid-state laser pumping. Therefore, highly efficient pumping may be achieved by utilizing double clad fibers as a gain material.
Modern high power pumping techniques for commercial fiber lasers and amplifiers are usually based on end or side pumping by diode lasers. The common double clad fibers used for fiber lasers applications are Yb3+ doped silica with tunable output between 980 nm-1200 nm (pumped by either 915 nm or 980 nm diodes), Er3+ doped silica for 1550 nm eye-safe and communication applications (pumped by either 980 nm or 1480 nm diodes), and Yb3+:Er3+ silica fibers used also for 1550 nm applications, but in the high power range, where the wide spread erbium doped fibers are not applicable. Other fiber lasers used mostly for 2 μm remote sensing and medical applications are Tm3+ doped and Ho3+:Tm3+ doped silica fibers.
The most commonly used fiber for marking, drilling and other industrial applications is the Yb3+ fiber, characterized by high efficiency and robustness. In addition, reliable and efficient pump diodes are available for this ion excitation, while its wide absorption band (25 nm) allows using pump diodes that do not need active cooling by a Peltier effect element or liquid chiller. The fiber's high efficiency and high surface-to-volume ratio enables cooling by air rather than cumbersome liquid cooling in solid-state lasers.
Previously, much of the work done on erbium-doped fiber amplifiers has concentrated on maximizing the small signal optical gain, which in turn requires a small “spot size” or mode-field diameter (MFD). This also provides single mode operation considered desirable in applications requiring a high beam quality, communication applications and applications requiring very short pulses [D. J. Richardson et al., “Fiber Laser Systems Shine Brightly”, Laser Focus World, September 1997, pp. 87-96].
When a low energy pulsed source should be amplified to high energy pulses by a fiber amplifier, the core diameter of the fiber amplifier is limited to a minimum value, under which the fiber amplifier performance deteriorates. The core diameter reduction may cause elevated power density flow in the fiber, which may stimulate fiber damage or non-linear effects such as stimulated Raman or Brillouin scattering or Self Phase Modulation, which have deleterious effect on the amplified pulse.
In one example, the maximum tolerable peak power in 1 m of a single mode previous doped optical fiber is about 500 W.
Similar problems can also occur in CW lasers and amplifiers where nonlinear effects such as Brillouin scattering can limit the output power when operating with narrow line widths (e.g. <10 MHz). For 1 m of conventional fiber in CW operation the nonlinear threshold for Brillouin scattering is about 20 W.
A further restriction on the available output power from pulsed fiber is the energy storage capacity of the amplifying fiber. The high gain coefficients in conventional single mode fibers limit the energy that can be stored to about 30 μJ. On the other hand, by using a fiber with a high diameter core that supports multi-mode operation in which the deleterious effects are eliminated, a very low gain and high amplification threshold are achieved, yielding a very low efficiency amplifier. Furthermore, when multi-mode fiber is used, a poor beam quality is achieved, which is unsuitable for applications demanding single mode or alternatively, near diffraction limited beam.
High energy pulses or high power CW single frequency (or alternatively, narrow line width) operation while keeping the beam quality close to the diffraction limit, has been achieved by various methods. U.S. Pat. No. 6,614,975 to Richardson, et al. uses special fiber arrangement for enlarging the core MFD so as to enable high energy pulses and CW single frequency, single mode amplification. U.S. Pat. No. 6,496,301 to Koplow, et al. induces single mode operation on multi-mode fibers by coiling the multimode gain fiber to induce significant bend loss for all but the lowest-order mode(s).
Reference is now made to FIG. 1, in which a prior art low energy source amplification in a high power fiber amplifier is illustrated. A high power diode 10 may pump optical power to a rare-earth doped double clad fiber 18 (e.g., Yb3+ doped fiber), utilizing the previously mentioned methods for achieving single mode high power, high energy operation, through coupling optics 12 and an end-fiber coupling section 14. A seeder 16, such as a 1.064 μm diode, may inject low power (in case of CW single frequency operation) or low energy signals (in case of pulsed operation), to coupling section 14. Coupling section 14 may be coated for anti-reflection at the pump wavelength and may have high reflection at the signal wavelength. The double clad fiber 18 may be connected to output coupling optics 19.
However, when low energy source amplification is required, these methods are inadequate for achieving highly efficient amplification, due to the fact that the signal amplification threshold is high (mainly because of the high saturation energy and power, resulting from the high MFD, manifested by these methods) and the high noise evolution, mainly due to the Amplified Spontaneous Emission (ASE), resulting from the high population inversion and low efficiency signal amplification.